1. Field of the Invention
The present invention relates to a resin-molded stator, a method of manufacturing the same, and a rotary machine using the same.
2. Description of the Prior Art
Conventional resin-molded stators and rotary machines using the same employ the structure where, as disclosed in Japanese Application Patent Laid-Open Publication No. 08-223866, Japanese Application Patent Laid-Open Publication No. 09-157440, and Japanese Application Patent Laid-Open Publication No. 10-51989, the stator coil wound around the plurality of slots or bumps in the stator core, and the stator coil at the end of the stator core are molded with resin and the molding resin and the end of the stator core that faces in the axial direction of the rotary machine are bonded. Also, an alternating-current (AC) power generator using brushes to supply power to the rotor is disclosed in Japanese Application Patent Laid-Open Publication No. 2000-125513.
The present inventors have found that the prior art practice poses the problem that when a stringent acceleration test simulating the operation mode in which the output of the rotary machine is to be increased from the level under its stopped status to the maximum achievable level within a short time is repeated several times, insulation breakdown occurs between the stator coils and the ground or between the different phases of the stator coils. It is considered that the insulation breakdown occurs as follows:
It has been examined why and how the insulation breakdown occurs between the stator coils and the ground or between the different phases of the stator coils when the operation mode in which the output of the rotary machine is to be increased from the level under its stopped status to the maximum achievable level within a short time is repeated several times. As a result, it has been found that when the output level is increased to its maximum within a short time, the stator coils are heated by the electrical resistance of the coil conductor and, depending on the particular conditions, the coil temperature increases to a maximum of about 250° C. At the same time, it has also been found that since the heat capacity of the stator core is high, increases in the temperature thereof retard with respect to the stator coils. Accordingly, the difference in temperature between the stator coils and the stator core often reaches 200° C. or more.
The thermal expansion coefficient of the copper used for the stator coils is 1.7×10−5 1/° C., and the thermal expansion coefficient of the stator core in the direction that electromagnetic steel plates were laminated in the direction of the rotational axis of the stator core to form the core is about 1.3×10−5 1/° C. In this way, the stator coils and the stator core differ in thermal expansion coefficient, and when a temperature difference exists between the stator coils and the stator core, the amount of thermal elongation also differs between both. The resin-molded stator may have the structure where one side of its coil end is constrained by being positioned between the stator core and the end face of the housing in the direction of its rotational axis in the rotary machine. The relative displacement Δ occurring between the stator coils and the stator core, at the coil end of a resin-molded stator having such structure, is represented by formula (1) below.Δ={αc×(Tc−Tr)−αf×(Tf−Tr)}×L  (1)where: “αc” and “αf” are the thermal expansion coefficients of the stator coils and the stator core, respectively; “Tc” and “Tf” are the temperatures of the stator coils and stator core when the output of the rotary machine is increased to the maximum output level within a short time; “Tr” is the temperature at which the difference between the stator coils and the stator core in terms of thermal elongation is zero, and; “L” is the laminating thickness of the stator core.
For example, if the laminating diameter of the stator core in a resin-molded stator having the structure where one side of its coil end is constrained by being positioned between the end face of the housing and the stator core is 100 mm, when the temperature of the stator coils increases from 20° C. to 230° C. and the temperature of the stator core increases from 20° C. to 50° C., the relative displacement Δ occurring between the stator coils and stator core at the coil end can be calculated by assigning, to formula (1) shown above, an “αc” value of 1.7×10−5 1/° C. as the thermal expansion coefficient of the stator coils, an “αf” value of 1.3×10−5 1/° C. as the thermal expansion coefficient of the stator core, a “Tc” value of 230° C. as the temperature of the stator coils, a “Tf” value of 50° C. as the temperature of the stator core, a “Tr” value of 20° C. as the temperature at which the difference between the stator coils and the stator core in terms of thermal elongation is zero, and an “L” value of 100 mm as the laminating thickness of the stator core. As a result, it follows from the difference in thermal elongation that the relative displacement Δ occurring between the stator coils and stator core at the coil end is 0.35 mm.
The stress “σc” applied to the stator coil section when the space between the stator core and the stator coils is constrained so as not to cause relative displacement between both can be represented using the following formula (2) which assumes that all thermal strain is imposed on the stator coils:σc=Δ×Ec/L  (2)where “Δ” is the relative displacement between the stator coils and stator core at the coil end, “Ec” is the longitudinal elastic modulus of the stator coils, and “L” is the laminating thickness of the stator core.
If “Δ”, “Ec”, and “L” are 0.35 mm, 100 GPa, and 100 mm, respectively, the stress “σc” applied to the stator coils reaches 0.35 GPa.
The stator coils running through the stator slots and emerging at the coil end are split into sections wound clockwise and counterclockwise around the stator core according to phase and engage with other stator slots. Accordingly, the coils wound from a plurality of stator slots towards other stator slots are accommodated under a mutual contact status at the coil end section of the stator. In this case, if the stator core and the resin-molded section at the coil end are bonded, a thermal elongation difference reaching 0.35 mm occurs between the stator core and stator coils at the coil end, and at the same time, since the stator coils emerging at the coil end are wound in different directions for each phase, a phase shift occurs between the coils of different phases and is likely to damage the insulation around the conductors, resulting in insulation breakdown. The same also applies to concentrated-winding structure having coils wound at the protrusions of the stator core. That is to say, if the stator core and the resin-molded section at the coil end are bonded, the difference in thermal elongation between the conductors and the core due to abrupt increases in the temperature of the conductors causes the buckling thereof and is likely to damage the insulation, and resulting in insulation breakdown.